1. Field
This invention pertains to removal methods for arsenic contaminants from neutral drinking waters. More particularly it provides an electrochemical insolubilization method and apparatus for the removal of arsenic contaminants from neutral drinking waters.
2. State of the Art
Various removal methods of heavy metal ions from aqueous solutions are known. For example, Bouard et al, U.S. Pat. No. 5,425,857 disclose a process and device for the electrolytic generation of arsine. Brewster, U.S. Pat. No. 5,368,703 discloses a process and apparatus for removing arsenic from aqueous media utilizing ferrous ion provided by an electrochemical cell which is added to the aqueous media in a first step, followed by a second step of mild oxidizing conditions generated by the addition of hydrogen peroxide (Fentons Reagent). The ferrous ion then forms a hydroxide that is oxidized to an oxyferric oxide and the arsenic present at a +3 oxidation state is oxidized to a +5 oxidation state. A precipitate is formed which consists of hydroxy ferric oxide to which the arsenic oxyacid is adsorbed (co-precipitated).
O'Connor et al., U.S. Pat. No. 5,182,023 discloses a process for treating arsenic-containing aqueous waste using ultra filters to remove solids. It is followed by a chemical treatment to adjust the pH range from about 6 to 8. Then, antiscalents and antifouling material are added. The chemically treated filtrate is then subjected to a reverse osmosis process to result in a permeate stream having less than about 50 parts per billion arsenic.
Gallup, U.S. Pat. No. 5,024,769 discloses a method of treating an aqueous solution containing one or more arsenic compounds in a +3 oxidation state. The method comprises contacting the aqueous solution with a halogenated organic oxidizing agent to convert the +3 arsenic to a +5 oxidation state.
McClintock, U.S. Pat. No. 5,358,643 discloses a treatment method for removing arsenic from water via conditioning the water with one or more additives including an iron salt, an acid, and an oxidant until the water contains more ion than arsenic, is acidic, and has an oxidation-reduction potential of about plus 600 mV. A reagent is then added to the conditioned water until it becomes basic and the water and additives are then reacted in a reaction chamber wherein iron and arsenic are coprecipitated in the form of iron arsenate and iron hydroxide or iron oxide. The treated water is then separated from the precipitate by settling or filtering.
In the article entitled "Use of Electrochemical Iron Generation for Removing Heavy Metals from Contaminated Groundwater" by M. D. Brewster and R. J. Passmore published in the Environmental Progress (Vol. 13, No2) May, 1994 at page 143, the authors discuss an electrochemical iron addition process developed by Andco Environmental Processes, Inc. The process employs electric current and steel electrodes to put ferrous and hydroxyl ions into solution. Once added, the chemistry is manipulated with the addition of various ions to provide coprecipitation and adsorption conditions capable of simultaneously removing a wide variety of heavy metals. For example, hydrogen peroxide was introduced to convert Fe+2 to Fe+3, and arsenite to arsenate. The pH was then adjusted to precipitate and adsorb various heavy metal oxides.
The method described below provides a continuous processing method to remove ions from an aqueous solution by: a) identifying the ion(s) to be removed and their oxidation state, b) identifying the pH of the solution, c) constructing an electrolytic cell with a corroding anode and an inert cathode surrounding a reaction zone therebetween, and d) utilizing an iron anode to electrochemically precipitate the desired ionic specie for physical removal of the precipitates from the aqueous solution. The method avoids the need for additional reagents and the removal problems associated therewith. It also reduces the ion concentration left in the treated aqueous significantly less than the other methods described above.